The present invention relates to an active matrix liquid crystal display device utilizing a switching element such as a thin film transistor (hereinafter called TFT), and the method for manufacturing the same, and more specifically, to a shading means for shading the switching element, and the method for manufacturing the same.
Liquid crystal display devices are known for their advanced characteristics such as being light-weight, having reduced thickness, and exhibiting low power consumption, and active research and development is performed in the field. A liquid crystal display comprises xe2x80x9cpixel elementsxe2x80x9d arranged in matrix, which are formed by placing liquid crystal molecules in between transparent electrodes. When an arbitrary voltage is provided between the transparent electrodes corresponding to each pixel element, the alignment of the liquid crystal molecules in the pixel element is changed, and the degree of polarization of the light passing through the liquid crystal is varied, which leads to controlling the transmission rate of the light. The liquid crystal display device is divided into two types based on operation principles, that is, the simple matrix type and the active matrix type. Since the active matrix liquid crystal display device utilizes active elements, TFTs, switching elements for individual pixel elements, independent signals can be transmitted to each pixel element, and the device provides improved resolution and a clear display image.
A TFT utilizing amorphous silicon thin film is often used as the switching element for the active matrix liquid crystal display device. Moreover, a recently proposed technique refers to a TFT that utilizes a polysilicon thin film formed either by heat treating an amorphous silicon thin film in a temperature over 600xc2x0 C., or by providing a laser crystallization in which a pulse laser (such as an excimer laser) is radiated to the thin film for recrystallization. The polysilicon thin film is advantageous in that it has higher mobility compared to the amorphous silicon thin film, which allows for not only the switching elements for the pixels but also the driving circuit for driving the switching elements of the pixels to be formed on the same substrate using the polysilicon TFT.
As mentioned above, the liquid crystal display device controls the transmission rate of the light passing through the liquid crystal by changing the degree of polarization of the light passing through the liquid crystal, but the device itself is not equipped with a light emitting member. Therefore, a light source of some sort must be provided to the device. For example, in the case of a transmission-type liquid crystal display device, a lighting device, a so-called light, is placed on the back side of the liquid crystal display, and the light transmitted through the device enables images to be displayed. In the case of a projector, a metal halide lamp and the like are used as the light source, and image is projected by combining the liquid crystal display device with a lens system. Moreover, in case of a reflection-type display, the incident light provided from the exterior is reflected by a reflecting electrode in order to display an image.
In general, if light is radiated to a semiconductor, such as silicon, and light absorption occurs, electrons are excited to the conductive band and positive holes are excited to the valence band, generating electron-hole pairs and causing a so-called photoelectric effect. The same could be said for the amorphous silicon thin film or the polysilicon thin film utilized as the pixel switching elements. By radiating light thereto, electron-hole pairs are generated in the thin film. Accordingly, when light is radiated to the TFT using either the amorphous silicon thin film or the polysilicon thin film as the active layer, photocurrent is caused by the electron-hole pairs, which increases the leak current during the off-state of the TFT. This leads to deteriorating the contrast and the like of the liquid crystal display.
In the case of a reflection-type liquid crystal display device, the reflecting electrode mainly composed of a metal film connected to the TFT is arranged to cover the TFT, so that no incident light from the exterior reaches the TFT directly. Accordingly, TFT leak current is prevented from increasing. However, in the case of a transmission-type liquid crystal display device, the TFT is not only exposed constantly to the strong light from the back light, but some incident light other than that from the back light also tends to reach the TFT. Moreover, in the case of projectors, the light reflected by the lens may reach the TFT. Accordingly, various inventions are proposed that aim at preventing incident light from reaching the TFT.
For example, as shown in FIG. 11, shading film 63 and shading film 64 are provided above and under the switching electrode 62 via insulation layers, in order to block the light coming from above and under the switching element (Japanese Patent Application Laid-Open Publication No. 58-159516). This is effective in reducing leak current, and in improving display characteristics.
According to another proposal, as shown in FIG. 12, in an adhered SOI substrate, an upper shading layer 66 and a lower shading layer 67 are provided above and under a MOSFET 65, in order to block the direct incident light coming from above and under the MOSFET, and to also block the light reflected by the back surface of the substrate, thereby effectively preventing an increase of TFT leak current (Japanese Patent Application Laid-Open Publication No. 10-293320).
According to yet another proposal, as shown in FIG. 13, by providing a shading film 69 under the switching element 68 and providing a black matrix 70 formed of silicon thin film and silicide film on the opposing substrate, not only the direct incident light is blocked, but also the reflection of light within the liquid crystal display device is restrained, since the fine unevenness provided to the surface reduces the reflection rate and diffuses light (Japanese Patent Application Laid-Open Publication No. 10-319435).
According to the above method, shading layers are provided above and under the TFT so as to prevent incident light coming in from the exterior from reaching the semiconductor film or active layer of the TFT, and most of the incident light fails to reach the semiconductor film. However, the incident angle of the light coming into the liquid crystal display device is not always perpendicular the substrate, but has a certain degree of dispersion, and the light entering the display device may be repeatedly reflected within the device. When light reaches the TFT according to these reasons, the light causes problems such as an increase of TFT leak current.
As shown in FIG. 10(a), light (A) and light (B) are blocked by the upper shading layer 54 and the lower shading layer 51, and they will not reach the TFT 55. However, the oblique incident light (C) coming from the side of upper shading layer 54 is reflected by the lower shading layer 51, and reaches the TFT 55. Moreover, the oblique incident light (D) coming from the side of upper shading layer 54 side is reflected by the lower shading layer 51, then reflected by the upper shading layer 54, before reaching the TFT 55. Similarly, the incident light (E) and (F) coming from the side of lower shading layer 51 also reaches the TFT 55 after being reflected one or more times. Therefore, according to the proposal of Japanese Patent Application Laid-Open Publication No. 58-159516, light traveling as mentioned above will reach the transistor causing an increase of leak current.
Moreover, as shown in FIG. 10(b), when the upper shading layer 60 is larger than the lower shading layer 57, the oblique incident light (C), (D) and (G) coming from the side of upper shading layer 60 is blocked by the upper shading layer, but on the other hand the oblique incident light (E), (F) and (I) coming from the side of lower shading layer 57 still reaches the TFT 61, and the oblique incident light (H) coming from the side of lower shading layer that would not have reached the TFT if the upper and lower shading layers were the same size also reaches the TFT 61 since it is reflected by the back surface of the upper shading layer 60. As mentioned above, according to the invention disclosed in Japanese Patent Application Laid-Open Publication No. 10-293320, light traveling as described above will reach the transistor causing an increase of leak current.
Moreover, according to the method indicated in Japanese Patent Application Laid-Open Publication No. 10-319435, the light (G) coming in from the side of upper shading layer 54 shown in FIG. 10(a), or the light (I) coming in from the side of lower shading layer 57 shown in FIG. 10(b) will be diffused by the fine unevenness of the surface of the shading layer, and some of the light that would have reached the TFT if not for the diffusion will be removed effectively. However, since the direction of light reflected by the uneven surface of the shading layer is random, the light that would have reached the TFT by the second reflection if the surface of the shading layer were smooth would reach the TFT by a single reflection. The described arrangement causes some light to reach the TFT more easily, thereby causing an increase of leak current, similarly to the other two prior art examples.
As explained above, it is difficult according to prior art techniques to prevent incident light traveling obliquely into the display device from above and under the device from reaching the TFT. By sufficiently increasing the size of the upper and lower shading films, it may be possible to reduce the intensity of the light reaching the TFT reflecting many times on the upper and lower shading films, by the reflection rate of the upper and lower shading films and the light absorption caused by the insulation film between the upper and lower shading films. However, according to such a method, the area of the shading films are insufficiently increased, causing other problems such as reduction of aperture rate, an important element of liquid crystal displays. Moreover, the increase of size of the shading films does not fundamentally prevent light from reaching the TFT.
The present invention aims at solving the above-mentioned problems. The object of the present invention is to provide an active matrix liquid crystal display having improved brightness and high contrast, and the method of manufacturing the same.
The present invention provides an active matrix liquid crystal display device comprising a liquid crystal cell, a switching element arranged in matrix, and shading layers mounted both on the upper side and the lower side of the switching element; wherein at least one of the upper and lower shading layers includes a sloped portion and has a convex shape protruding toward the switching element.
The present invention also provides an active matrix liquid crystal display device comprising a liquid crystal cell, a switching element arranged in matrix, and shading layers mounted both on the upper side and the lower side of the switching element; the upper shading layer including an upper sloped portion and having a convex shape protruding toward the switching element, the lower shading layer having a flat shape: wherein the upper shading layer is formed so that the upper sloped portion is located at a xcex81 angle to the horizontal direction, and the upper sloped portion has a horizontal direction length of l11; the lower shading layer is formed so that the length from the end of the lower shading layer to the point that the line drawn downward to the vertical direction from the origin of the upper sloped portion crosses the lower shading layer is l12; and the maximum incident angle of the incident angle of the light traveling obliquely from the upper shading layer side is xcex11, the maximum incident angle of the light traveling obliquely from the lower shading layer side is xcex21, and the distance between the upper shading layer and the lower shading layer is d1, in which xcex81, l11 and l12 each fulfill xcex81 greater than xcex21, l11 greater than (l12+d1xc2x7tan xcex11)/(1xe2x88x92tan xcex81xc2x7tan xcex11), and l12 greater than d1xc2x7tan xcex21.
Moreover, the present invention provides an active matrix liquid crystal display device comprising a liquid crystal cell, a switching element arranged in matrix, and shading layers mounted both on the upper side and the lower side of the switching element; the lower shading layer including a lower sloped portion and having a convex shape protruding toward the switching element, the upper shading layer having a flat shape: wherein the lower shading layer is formed so that the lower sloped portion is located at a xcex82 angle to the horizontal direction, and the lower sloped portion has a horizontal direction length of l21; the upper shading layer is formed so that the length from the end of said upper shading layer to the point that the line drawn upward to the vertical direction from the origin of the lower sloped portion crosses the upper shading layer is l22; and the maximum incident angle of the light traveling obliquely from the lower shading layer side is xcex12, the maximum incident angle of the light traveling obliquely from the upper shading layer side is xcex22, and the distance between the upper shading layer and the lower shading layer is d2, in which xcex82, l21 and l22 each fulfill xcex82 greater than xcex22, l21 greater than (l22+d2xc2x7tan xcex12)/(1xe2x88x92tan xcex82xc2x7tan xcex12), and l22 greater than d2xc2x7tan xcex22.
Further, the present invention provides an active matrix liquid crystal display device comprising a liquid crystal cell, a switching element arranged in matrix, and shading layers mounted both on the upper side and the lower side of the switching element; the upper and lower shading layers respectively including an upper sloped portion or a lower sloped portion, both having a convex shape protruding toward the switching element, and the lower sloped portion formed longer than the upper sloped portion: wherein the upper shading layer is formed so that the upper sloped portion is located at a xcex831 angle to the horizontal direction, and the upper sloped portion has a horizontal direction length of l31; the lower shading layer is formed so that the lower sloped portion is located at a xcex832 angle to the horizontal direction, and the lower sloped portion has a horizontal direction length of l32; and the maximum incident angle of the light traveling obliquely from the upper, shading layer side is xcex13, the maximum incident angle of the light traveling obliquely from the lower shading layer side is xcex23, and the distance between the upper shading layer and the lower shading layer is d3, in which xcex831, xcex832, l31 and l32 each fulfill xcex831 greater than xcex23, xcex832 greater than xcex13, l31 greater than tan xcex23xc2x7(d3+l32xc2x7tan xcex832), and l32 greater than tan xcex13xc2x7(d3+l31xc2x7tan xcex831).
Moreover, the present invention provides an active matrix liquid crystal display device comprising a liquid crystal cell, a switching element arranged in matrix, and shading layers mounted both on the upper side and the lower side of the switching element; the upper and lower shading layers respectively including an upper sloped portion or a lower sloped portion, both having a convex shape protruding toward the switching element, and the upper sloped portion formed longer than the lower sloped portion: wherein the lower shading layer is formed so that the lower sloped portion is located at a xcex841 angle to the horizontal direction, and the lower sloped portion has a horizontal direction length of l41; the upper shading layer is formed so that the upper sloped portion is located at a xcex842 angle to the horizontal direction, and the upper sloped portion has a horizontal direction length of l42; and the maximum incident angle of the light traveling obliquely from the lower shading layer side is xcex14, the maximum incident angle of the light traveling obliquely from the upper shading layer side is xcex24, and the distance between the lower shading layer and the upper shading layer is d4, in which xcex841, xcex842, l41 and l42 each fulfill xcex841 greater than xcex24, xcex842 greater than xcex14, l41 greater than tan xcex24xc2x7(d4+l42xc2x7tan xcex842), and l42 greater than tan xcex14xc2x7(d4+l41xc2x7tan xcex841).
According to another aspect of the invention, in the above liquid crystal display devices, the upper shading layer and the lower shading layer are each formed of one of the following: a metal film (Al, Ta, Ti, W, Mo, Cr, Ni), a singled layered film made for example of polysilicon, AlSi, MoSi2, TaSi2, TiSi2, WSi2, CoSi2, NiSi2, PtSi, Pd2S, HfN, ZrN, TiN, TaN, NbN, TiC, TaC or TiB2, or of a structure formed by laminating said films.
Even further, the present invention provides a liquid crystal display device according to the above, wherein either the upper shading layer or the lower shading layer, or both said upper and lower shading layers, is or are also used for wiring.
Moreover, the present invention provides a method for manufacturing the liquid crystal display device according to any disclosed above, wherein the layer underneath either the upper shading layer or the lower shading layer is formed using SiO2, which is isotopically etched through HF using a resist mask, and removed of the mask before either the upper shading layer or the lower shading layer is formed thereon.
Lastly, the present invention provides a method for manufacturing the liquid crystal display device according to any disclosed above, wherein the layer underneath either the upper shading layer or the lower shading layer is formed using SiO2, which is isotopically dry-etched using a resist mask, and removed of the mask before either the upper shading layer or the lower shading layer is formed thereon.